In magnetic recording technology, desired data is generally recorded by magnetizing the surface of a fixed disk. The disk used in the magnetic disk apparatus inevitably has a rough disk surface and nonuniform magnetic material to some extent. It is desired to improve the density at which information can be recorded and read. The total memory storage capacity or recording density is proportional to the number of tracks and the linear density on the surface of a disk. The number of tracks is dependent upon the width of the tracks.
In order to achieve more narrow tracks, the need to more precisely control the surface conditions of the disks has become increasingly important. A defect or bump on the surface of the disk can degrade or grow, thereby resulting in an unuseable area on the disk. There is a problem of growing defects over the life of a media. A growing defect is a defect on the media that enlarges over time due to physical degradation of the media. The media can become damaged due to dust, scratches, etc. Areas that are not used often may have growing defects due to head movement across the surface during a seek. Moreover, any unwanted contact between the head and magnetic surface can also lead to wear and the accumulation of debris on the disk causing random signal modulations. Typically, during non-use of the drive (e.g., after the drive is idle for 1 second), the heads just sweep or move over media without any reading or writing. This prevents the heads from staying in one location which would otherwise lead to media wear at that location.
Flaws or defects in the disk surface manifest themselves as missing bits or extra bits of data (i.e., single bit errors). Missing bits are reductions in the amplitude of the envelope of the signal, usually over a small number of bits (e.g., 1-4 bits), such that the amplitude falls below a channel-detector clipping level. The number of missing bits observed depends on the setting of the clipping level in the readback channel.
Several methods exist for detecting defects on the surface of a disk. One method involves performing media verification on the disk surface prior to packaging the disk and shipping it to the consumer or end user. However, this technique only locates the defects that exist originally, and will not locate defects that form after shipment of the disk and during usage. Another technique that can be used is for the end user to reformat the entire disk. This will locate any existing defects. However, this technique erases the existing data, so that the data must be saved elsewhere during the reformatting. This is very inconvenient for the user. Furthermore, typically, when data is read from or written to a disk, the media is checked at the critical locations with error correction coding (ECC). In this manner, heavily used areas are checked often for defects. However, areas that are not used frequently are not checked often for defects. Thus, at these locations, growing defects can form and grow without early detection.
Although the art of error detection on media is well developed, there remain some problems inherent in this technology, particularly early detection of growing defects. Therefore, a need exists for a system and method of detecting growing defects that overcomes the drawbacks of the prior art.